In zoom objective lenses having a plurality of lens groups, a separate front lens group is conventionally configured as a focus group and is moved during focusing. This separate focus group operates independently from those lens groups which vary the image scale during zooming. If no further measures are taken, the movement of the focus group during focusing of the image causes the image angle (also referred to as “image field angle”) to vary.
In order to avoid this phenomenon, various measures have already been proposed, according to which the focus group is subdivided into two lens groups which are moved according to certain rules.
Document DE 79 08 085 U1 mentioned at the outset discloses several such concepts in which these two lens groups are arranged in front of the iris diaphragm of the objective lens. According to a first variant, one lens group of the focus group is held stationary and only the other one is moved. According to a second variant, both lens groups are simultaneously moved during focusing, however, at different speeds. When doing so, the position of a rear main plane of the entire lens system remains unchanged.
In an objective lens according to document U.S. Pat. No. 4,278,331 the two lens groups of the focus group being likewise positioned in front of the iris diaphragm, are moved in opposite directions relative to one another, such that the angle between a light beam directed onto the entrance pupil of the objective lens and the optical axis of the objective lens remains constant during focusing.
Document U.S. Pat. No. 4,336,983 describes another objective lens having means for avoiding a variation of the image angle during focusing. In this objective lens only one lens group being arranged in front of the iris diaphragm is moved during focusing. Two further lens groups are not moved during focusing. An iris diaphragm is provided between the second and the third lens group. The focal point of the combination of the second and the third lens group coincides with the exit pupil of the entire lens system.
The problem of avoiding variations of the image angle during focusing is also the subject of scientific publications, e.g. K. Tanaka “A Zoom Lens without Focus Breathing Phenomena”, Proc. of SPIE Vol. 4487 (2001), pages 63-67; I. Neil “High Performance Wide Angle Objective Lens Systems with Internal Close Focusing Optics and Multiple Aspheric Surfaces for the Visible Waveband”, SPIE, Vol. 2774, pages 216-242; I. Neil “High Performance, Wide Angle, Macro Focus, Zoom Lens for 35 mm Cinematography”, SPIE Vol. 3482, pages 213-228.
The prior art objective lenses of the type specified before, have the disadvantage that due to the positioning of the moved focus group in front of the iris diaphragm negative influences occur with regard to the correction of the objective lens.